The present invention relates to an air-fuel ratio control apparatus for controlling the air-fuel ratio of an engine depending on an oxygen concentration in exhaust gas detected by an oxygen sensor.
There are various known oxygen sensors including a sensor (a .lambda. point oxygen sensor) in which the output value greatly changes in the vicinity of the theoretical air-fuel ratio so that an air-fuel ratio in the vicinity of the theoretical air-fuel ratio is detected, and a sensor (a wide range oxygen sensor) which generates an output value for detecting an air-fuel ratio in a wide range from a lean state to a rich state.
A control apparatus controls the air-fuel ratio of an engine by performing a feedback correction depending on an air-fuel ratio detected by an oxygen sensor.
Since such an oxygen sensor is exposed to exhaust gas for a long period, a value of the detected air-fuel ratio may vary due to a performance deterioration of the oxygen sensor caused by a thermal and or a chemical attack of exhaust gas.
In a specific example, as shown by a solid line B in FIG. 10, in a .lambda. point oxygen sensor, the air-fuel ratio at which the output is greatly changed (the theoretical air-fuel ratio point to be detected) is shifted to the richer side as the sensor is used for a long period. In a wide range oxygen sensor, as shown in FIG. 11 , the output (a pump current) decreases from a value shown by a solid line C to a value shown by a one-dot chain line D as the sensor is used for a long period.
In this way, the value of the air-fuel ratio detected by an oxygen sensor varies as the oxygen sensor is used for a long period. Therefore, a prior art air-fuel ratio control apparatus cannot properly control the air-fuel ratio of an engine for a long period.